Fluorescent lamps are well known in the art and are used for a variety of types of lighting applications. Such lamps are characterized as low pressure arc discharge lamps and include an elongated envelope, whose internal surface is coated with phosphor, an electrode structure at each end of the envelope. The envelope also contains a quantity of an ionizable material, such as mercury, and a fill gas at a low pressure, for example in the order of 1-5 torr. The fill gas can be, for example argon or krypton, or a mixture of these and other gases.
When a voltage is applied across the electrodes, electrons will be emitted, ionizing the gas inside the envelope. The resultant ionization and recombination of ions and electrons produce 253.7 nm. radiation which is converted by means of the phosphor into radiation of a longer wavelength and a spectral distribution, depending on the phosphor material used, in the near ultra-violet or in the visible part of the spectrum.
One such lighting application in which fluorescent lamps are extensively used is in reprographic equipment for photocopying. Equipment used in today's reprographic technologies utilizes varied illumination and optical processes to accomplish the desired duplicating process. The fundamentals of the electrophotographic cycle are: a grid-controlled primary charger, an organic photo-conductive film belt, and a semi-solid development process.
Several different illumination sources are used in these processes. The type of source and its location are dependent on the manufacturer, make, and model of the specific machine. Most reprographic equipment, however, does include the use of at least one fluorescent light source. In some copiers, a red 12-watt fluorescent light source is utilized in three different parts of the reprographic process. First, the auxiliary erase which follows the exposure area. During this operation, the light is used to expose the photoconductor in order to remove any unwanted charges. Second, the post development erase wherein light is used to expose any remaining charges on the film. This process reduces the electrical stress on the photoconductor. Third, the cleaning assist area which has the same configuration as the post development erase area.
Fluorescent lamps which are designed to emit a selected color are well known in the art and achieve the desire color-controlled light output by employing suitable combinations of phosphors and light-filtering materials. For example, in the case of a red-emitting fluorescent lamp designed for use as a light source for a photographic dark room or as a decorative lamp, a phosphor coating, that emits mainly in the red region of the spectrum in response to the ultraviolet radiations produced by the arc discharge, is combined with an underlying layer of red pigment that filters out all of the visible radiation below a certain wavelength (600 nanometers, for example). A prior art fluorescent lamp which utilizes this color-controlling concept and combines a suitable phosphor and a filter material to provide a lamp which emits mainly in the red region of the spectrum is disclosed in U.S. Pat. No. 3,365,232 which issued to Repsher on Jan. 23, 1968.
While the transmitted radiation provides the pure red light which is desired, the use of thin layers of pigment material in fluorescent lamps inherently result in lamp-starting problems. In order to avoid such problems, prior art lamps include various additives in the phosPhor to facilitate starting of the fluorescent lamp. In the case of a red-emitting fluorescent lamp that utilizes a thin layer of cadmium-selenium sulfide pigment as the filter component together with a phosphor which has an emission mainly in the red portion of the spectrum, a prior art practice was to add a mixture of barium sulfate and silica to the phosphor as a starting aid in amounts that frequently exceeded 40% by weight or more of the phosphor coating. The addition of such additives has been found to not completely solve the lamp-starting problem. Since such additives are inert (non-fluorescent), they reduce the "red light" output of the lamp.
Other lamps, such as described in U.S. Pat. No. 4,500,810 which issued to Graff on Feb. 19, 1985, include an additional layer of a conductive material (such as tin oxide) on the inner surface of the glass envelope. Overlying layers of a light-filtering pigment material and a phosphor are disposed over the conductive layer. The addition of a layer of conductive material to improve lamp starting adds to the manufacturing complexity of the lamp.
Manganese-activated cadmium metaborate phosphor has been used in the past in conjunction with a cadmium-based red pigment filter coat. Because of increased restrictions on the use of cadmium, it is desirable to have a lamp source suitable for reproductive applications which is free of cadmium.
U.S. Pat. No. 4,547,700, which issued to Landry on Oct. 15, 1985 and is assigned to the same Assignee as the present application, discloses the use of alumina in a dual-layer fluorescent lamp to increase the lumen maintenance. The alumina, which is included within the second phosphor layer, is in an amount from about 6 to 50 percent by weight of the second phosphor layer.